1. Field of the Invention
The present invention relates to a novel easily-cuttable tunnel segment structure used so that a shield machine can depart from a main tunnel or return to the main tunnel.
2. Description of the Related Art
When a branch tunnel branched from a main tunnel is constructed, the operational or safety risk becomes larger as the depth of a job site increases in the prior art and when, for example, carried out at an entrance of a vertical shaft for the start or arrival of the shield machine for the purpose of solidifying the ground or demolishing a wall of an opening part in the main tunnel. Particularly, when the shield machine departs upward, the end cutting is extremely difficult and dangerous.
Also, there is a branch shield construction method wherein a branch shield machine for constructing a branch tunnel is built into a main shield machine for constructing a main tunnel and departs from the main tunnel. However, according to this method, a space within the tunnel is restricted and the operation is limited to the construction of only one branch.
In this connection, as a prior art in which the shield machine directly departs from or directly arrives at the main tunnel after the main tunnel has been constructed by using easily-cuttable segments, the Japanese Patent No. 2751636 discloses the invention of a segment for a shield tunnel, manufactured by using cuttable chord members of carbon, aramid, glass or vinylon staple fibers or filaments impregnated with resin as a reinforcement material for concrete. However, in the above-mentioned Patent Publication, there is no disclosure necessary for putting this invention into practice as a basic concept; that is, there is no concrete structure or means for bonding a plurality of segments into an integral body.
On the other hand, the above-mentioned fiber-reinforced cuttable structure for the direct departure or arrival of the shield machine has already been widely used as various retaining walls such as a continuous underground wall, SMW or a caisson. When prefabricated concrete products are applied to such uses, however, they are used as piles arranged at a predetermined gap which is usually filled with concrete or mortar in a job site. In other words, a structure has not been obtained in which the fiber-reinforced prefabricated concrete pieces are tightly bonded together without gap to form a continuous shield opening solely thereby. Accordingly, it has been difficult to apply such a prior art, as it is, to a case wherein the shield machine directly departs from or arrives at the main tunnel.
As described above, the prior art invention does not propose concrete means for achieving the object of the direct departure of the shield machine from the main tunnel or the direct arrival thereof at the main tunnel by cutting the segment structure without demolishing the already constructed tunnel wall. Particularly, when it is necessary for the shield machine to depart upward from the main tunnel, the execution thereof is extremely difficult.
In this regard, there are the following problems to be solved in a case wherein the easily-cuttable portion is provided in the tunnel segment structure to form a branch of the main tunnel, from which the shield machine departs or at which it arrives.
1. Bonding of a Plurality of Segments                Generally speaking, when the shielding is carried out, segment pieces, prepared by dividing a curved surface forming a wall of the tunnel in the circumferential direction and the longitudinal direction, are assembled together in a rear part of the tunneling machine. In this case, to apply a cuttable structure to the wall of the main tunnel for the purpose of the departure or arrival of the tunneling machine for forming the branch tunnel, it is necessary to provide a segment formed of a cuttable material in an area somewhat larger than a diameter of the tunneling machine for forming the branch tunnel.        Generally speaking, a longitudinal length of the segment is approximately in a range from 1.0 to 1.5 m, while an outer diameter of the tunneling machine for the branch tunnel exceeds 2 m. Therefore, the cuttable segments provided in the main tunnel for the departure or arrival of the branch tunneling machine extend over a plurality of rings, which requires bonding between the segments and the rings.        All the bonded portions between the segments and the rings also have a structure cuttable by the tunneling machine. In addition to this, it is necessary that, until the branch tunnel is cut open, the adjacent segments are brought into tight contact with each other without a gap as a wall of the main tunnel similar to ordinary segments forming a permanent structure. Also, it is necessary that a non-cut portion of the segment has the strength of a permanent structure.        
2. Water-Proofness                To prevent underground water from entering from the ground upon the departure or arrival of the shield machine, it is necessary to provide an entrance part into which a portion of the shield machine from a face plate to a front portion of a skin plate can penetrate.        However, this entrance part is not connectable in advance to the segment due to the spatial restriction when the easily-cuttable segment is assembled into the main tunnel. Accordingly, it is desirable to provide a structure in the segment for forming a base portion to be bonded to the entrance part after the segment has been assembled.        On the other hand, when crack generates during the cutting operation in the concrete of the cuttable segment which is put into practice, by the tunneling machine, there is a risk in that pressurized muddy water in the chamber of the shield machine or underground water in the background may enter the main tunnel via the crack. Thus, a structure and means for preventing such a problem is necessary.        